There are numerous applications where it is desirable to detect the presence of a person, animal, vehicle or other movable entity (sometimes hereinafter referred to as an "entity", "person" or "individual") within a selected three-dimensional space, motion of the entity within such space and/or the rate or speed at which and the direction in which the entity is moving through such space. One such application is discussed in copending application Ser. No. 07/749,279, filed Aug. 23, 1991 now U.S. Pat. No. 5,240,455. In this application,i to save energy and for other purposes, a laboratory fume hood is normally operated at a low face velocity. However, when a person comes within a selected area or space in front of the hood, such presence and/or motion is detected and the face velocity of the fume hood is automatically increased to enhance hood containment and to minimize any likelihood of the individual inhaling or coming in contact with a contaminant within the hood. Other applications where personnel detection or other movable entity detection within a selected space are desired include various security systems, including machine security systems which might close off a dangerous opening or enable some other safety feature when a person is within a selected area of the machine, animal confinement or control system, etc.
While many such detection systems are currently available, these systems generally suffer from a number of limitations. First, pyro-electric infrared systems detect changes in heat patterns caused by movement of a person or animal relative to background radiation in a selected space. However, since such systems are only adapted for detecting movement of an entity, if the entity stands still in the area, they blend into the background and are not detected. For this reason, a reliable determination that a person is no longer in the protected space cannot be made until no motion has been detected for a substantial time period, for example, five minutes. These systems thus have a slow response time to a person leaving the area, resulting in, for example, the fume hood application in the fume hood being in the more expensive high velocity mode for unnecessarily long periods of time.
Therefore, for applications such as control of a fume hood, near infrared or active systems are preferred, such systems detecting reflections of a transmitted infrared beam (or other radiation) from the selected space, or the absence of such reflections. One problem with near infrared systems currently in use is that they generally employ a single transmitter and a single receiver which are focused to a particular point within the space rather than providing broad coverage of the entire space. While the beam can be scanned to cover the entire area over some time period, such scanning is typically in only one direction, providing a band or zone within the space of interest which is being monitored, but not monitoring the entire space of interest.
A second problem with such systems is that, if the person is wearing clothing which is highly absorptive of the radiation being transmitted, for example black cotton for infrared radiation and animal fur for ultrasonic radiation, there will be no reflections from such entity entering the space. In many systems, this absorption creates the so-called "stealthperson" phenomenon which results in a lack of detection. The severity of this problem will depend to some extent on the radiation being used, not being a problem for microwaves which are not readily absorbed, and being much more of a problem for infrared and ultrasonic radiation. However, for reasons of cost, safety to individuals being scanned, and other reasons, infrared radiation is the preferred radiation for at least personal detection at this time.
A related problem results from the fact that an individual is detected by changes in reflection caused by the individual as opposed to the background. The changes in, reflected radiation result from the fact that, at the point being scanned, the reflection/absorption ratio for the individual is different than that for the background. However, if for example the individual is wearing a shirt which, at the distance the individual is from the emitter/detector, has the same reflective coefficient as the background, and a narrow scan focuses only on such shirt, the system may not be able to distinguish the individual from the background. Even if a larger area of the individual is scanned, if for example the individual is wearing dark pants and a white shirt, the average reflection from the individual in a particular scan direction may still be substantially the same as the background. Since to avoid spurious detections resulting from normal changes in background reflections over time (resulting from changes in ambient light conditions and other factors), detection of an individual in the selected space is normally not recognized unless a change in reflected radiation is detected exceeding a predetermined threshold, any small changes in reflected radiation from an individual having an average reflection/absorption pattern at the scan angle which substantially matches that of the background would not exceed the threshold, and the presence of the individual would, therefore, not be detected.
Another potential problem, particularly if light or other radiation is being received from a large area rather than from a single point, is that there may be substantial ambient radiation, for example sunlight or other ambient light, in the selected space and this ambient radiation may saturate the detectors so as prevent detection of an entity. While fail-safe procedures can be provided to assure detection when saturation occurs, this defeats the purpose of the entity detection system and it is preferable if such saturation can be avoided.
A related problem is that detection is based on a change in reflection pattern from background conditions. However, either because of movement of furniture or equipment in the space or other factors, the background radiation detection pattern may change. Most current systems are not adapted to automatically react to such changes in background conditions and, therefore, may not provide reliable detection. In particular, such systems may continue to provide a presence indication because of the background change even though there is no person in the selected space.
The above problem results from the fact that current near infrared systems detect only presence. It is therefore desirable in at least some applications to not only detect presence in the space, but to also detect motion. Detection of the direction and/or rate or speed of such motion are also desirable. Thus, action may be different if somebody is standing still in a protected area or is moving through an outer portion of the area in a direction parallel to a protected element than if a person is rapidly moving toward the protected element. Existing near infrared systems, and other systems of this type, are not generally capable of providing a motion indication and, in particular, are not capable of providing information concerning direction or speed of motion.
A need therefore exists for improved detection systems for persons or other movable entities, which systems (a) are capable of detecting both presence and motion throughout a relatively large three-dimensional space; (b) are able to compensate for changes in background condition from such space; (c) are able to detect "stealthpersons"; (d) are able to detect individuals having reflection/absorption patterns which may, at at least some distances and angles, substantially match the background absorption/reflection patterns; (e) permit detection of an individual in relatively high ambient light or other radiation conditions by avoiding saturation of the detector; and (f) have the capability for detecting direction and speed of motion.